1. Perianth bristles ± straight (or some with 1-2 kinks), slightly if at all exceeding achene (less than twice as long), minutely retrorsely barbed, sometimes rudimentary or absent; spikelets grouped into numerous hemispherical to spherical heads, each consisting of 5-25 or more spikelets (or some on a plant as few as 3); plants mostly solitary or few together, or colonial from long-creeping rhizomes; base of involucre not blackened.

2. Leaf sheaths all green; plants without long creeping rhizomes, in small clumps or solitary; bristles (often rudimentary) barbed only on apical half or third; styles 3-cleft; midrib narrow and green, excurrent on most scales as a short but distinct mucro; summit of culm smooth or slightly scabrous.

3. Perianth bristles absent or 1-3 and vestigial or much shorter than the achenes, with barbs lacking or only at tips.

S. georgianus

3. Perianth bristles 4-6, slightly shorter than to somewhat exceeding the achene, barbed for much of their length.

4. Bristles ± straight or stiffly curved to follow the achene outline, at least the longer definitely slightly exceeding the achene; lower leaf sheaths and blades distinctly septate-nodulose.

S. atrovirens

4. Bristles weak and contorted, all shorter than the achene; lower leaf sheaths and blades weakly septate-nodulose.

S. hattorianus

2. Leaf sheaths strongly tinged with red, at least toward base of plant; plants colonial from long-creeping rhizomes; bristles barbed nearly to their bases; characteristics of styles, scales, and culms not combined as above.

5. Styles 3-cleft; achenes 3-angled; bristles (3-) 6; summit of culm usually scabrous.

S. expansus

5. Styles 2-cleft; achenes 2-sided; bristles 4 (-5); summit of culm usually smooth.

S. microcarpus

1. Perianth bristles much crinkled or curled at maturity, exceeding the achene, smooth; spikelets mostly solitary or in groups of 2-5 on pedicels; plants usually in dense, many stemmed clumps; base of involucre blackened or not.

6. Bristles inconspicuous, scarcely exserted beyond the scales, about twice as long as achene; spikelets cylindric, mostly solitary on pedicels of various lengths; scales orange-brown with conspicuous green midrib excurrent as a very short awn.

S. pendulus

6. Bristles conspicuously exceeding the scales and achenes at maturity; spikelets short-ovoid to nearly spherical (very rarely cylindric), solitary on pedicels or in groups of mostly 2-5; scales various but midrib not conspicuously excurrent.

7. Spikelets all or mostly sessile in clusters of (2-) 3-7 or more.

S. cyperinus

7. Spikelets mostly pediceled, the ultimate branches of the inflorescence typically bearing 1 central, sessile spikelet with 2-3 pediceled ones.

8. Scales and bases of bracts dark blackish green; plants slender with leaves 2-5 mm wide.

S. atrocinctus

8. Scales and bases of bracts brown or gray-brown; plants more robust with leaves 3-10 mm wide.

S. pedicellatus